1. Field of the Invention
The present invention relates to an ejection head, an image forming apparatus, and an image forming method, and more particularly, relates to a structure of an ejection head and ejection control technology that reduce the visibility of unevenness caused in groups of dots formed on an ejection receiving medium (print medium).
2. Description of the Related Art
In recent years, inkjet recording apparatuses have come to be used widely as data output apparatuses for outputting images, documents, or the like. By driving recording elements, such as nozzles, provided in a recording head in accordance with data, an inkjet recording apparatus is capable of forming data onto an ejection receiving medium (recording medium), such as paper with ink ejected from the nozzles.
In an inkjet recording apparatus, a desired image is formed on a print medium by causing a print head having a plurality of nozzles and a print medium to move relatively to each other while ink droplets are ejected from the nozzles.
The inkjet head used in an inkjet recording apparatus may be a full line head having at least one nozzle row of a length corresponding to the full width of the print medium, or a serial head that forms a dot row in a main scanning direction by scanning with a short head, which has a shorter length than the full width of the print medium, in the breadthways direction of the print medium (main scanning direction). The full line head is able to print onto the full area of the printable region of the print medium by scanning once the print medium, by moving the head and the print medium relatively to each other in a sub-scanning direction substantially perpendicular to the width direction of the print medium. Therefore, the full line head is able to print at higher speed than the serial head.
FIGS. 22A, 22B, and 23 show a nozzle arrangement in a full line type print head according to the related art. FIG. 22A is a planar perspective view showing an example of the structure of a print head 500, and FIG. 22B is a planar perspective view showing another example of the structure of a print head 500.
In order to increase the density of the dot pitch printed onto the recording paper and to thereby improve print quality, it is necessary to increase the density of the nozzles by decreasing the nozzle pitch in the print head 500. The print head 500 shown in FIG. 22A achieves an apparent high-density of the nozzles by adopting a structure in which a plurality of ink chamber units 53 are arranged in a matrix configuration. Each of the ink chamber units 53 has a pressure chamber 52, whose planar shape is a substantially square and which is provided with a nozzle 51 and a supply port 54 at the respective corners on a diagonal of the planar shape thereof.
Moreover, as shown in FIG. 22B, it is also possible to use short heads 500′ that are arranged in a two-dimensional and staggered fashion, and are combined with each other, whereby the head can have a length corresponding to the full width of the print medium.
FIG. 23 shows the details of a nozzle arrangement shown in FIGS. 22A and 22B.
According to FIG. 23, the nozzles 51 are arranged in a lattice fashion in accordance with a uniform arrangement pattern following a row direction aligned in the main scanning direction, and an oblique column direction having a prescribed non-perpendicular angle θ′ with respect to the main scanning direction. By adopting a structure wherein a plurality of ink chamber units 53 are arranged at a uniform pitch d in a direction having an angle θ with respect to the main scanning direction, the pitch P0 of the nozzles projected to align in the main scanning direction (hereinafter referred to as projected nozzle pitch in the main scanning direction) will be d×cos θ.
More specifically, concerning the main scanning direction, the arrangement can be dealt equivalently to one in which the respective nozzles 51 are arranged in a linear fashion at a uniform pitch P0. By means of this composition, it is possible to achieve a nozzle composition of high density, wherein the nozzle rows projected to align in the main scanning direction reach a total of 2400 nozzles per inch.
Taking one example of the general dimensions of the print head 500 shown in FIGS. 22A, 22B, and 23, the size of the pressure chambers 52 is 700 μm×700 μm, the size of the actuators 58 is 500 μm×500 μm, the pitch P1 between nozzles that are mutually adjacent in the main scanning direction (the pitch between nozzles that eject droplets at the same timing) is 0.846 mm (corresponding to 30 nozzles per inch (npi)), the nozzle-to-nozzle pitch in the sub-scanning direction is 1 mm, and the ink viscosity used generally for the print head is 2 cp to 3 cp. Regarding the print head having such a structure, a variety of inventions have been suggested in order to increase density of nozzles and/or to improve printing quality.
In the inkjet printer described in Japanese Patent Application Publication No. 2002-166543, pressure chambers are formed to an approximate diamond shape or an approximate rectangular shape, in such a manner that a large number of ink pressure chambers corresponding to a plurality of nozzles can be arranged within the head of the inkjet printer.
In the inkjet head described in Japanese Patent Application Publication No. 2001-334661, the planar shape of the surface on which a pressurization plate of a chamber is arranged is defined to be an approximate square shape or an approximate diamond shape, in such a manner that the chambers are arranged at a high density.
In a head assembly method for a line type inkjet printer described in Japanese Patent Application Publication No. 2002-337320, matrix type heads are arranged adjustably in a staggered matrix fashion, in such a manner that printing errors corresponding to the joints between the heads are avoided.
In the inkjet recording apparatus described in Japanese Patent Application Publication No. 7-17034, recording heads in which the nozzles are driven in a split fashion are installed in a staggered fashion in the main body of the recording apparatus in such a manner that the boundaries for split driving of the nozzles do not overlap with each other. The optical density distributions of the images formed by the respective recording heads are mutually superimposed in such a manner that unevenness in density are alleviated and high image quality can be obtained.
In the inkjet printer apparatus and the print head unit described in Japanese Patent Application Publication No. 8-25635, in a long inkjet printer head formed by connecting together a plurality of heater boards, the heater boards are arranged in a staggered relationship at a prescribed distance apart in one uniform direction, in such a manner that unevenness in density occurring in regions corresponding to the boundaries between heater boards is reduced.
Next, the problems of the related art are described with reference to FIGS. 24 to 27.
FIG. 24 shows the relationship between the nozzle arrangement and the droplet ejection timing in a print head according to the related art. The numbers stated inside the circle indicating nozzles 51 show the droplet ejection timing, and at timing t1, droplets are ejected from the nozzles 51-11, 51-21, and so on, marked with the number 1. Then, at timing t2, droplets are ejected from the nozzles 51-12, 51-22, and so on, marked with the number 2. In this way, at timings t3 to t6, droplets are ejected from the corresponding nozzles, 51-13, 51-23, . . . , 51-16, 51-26, . . . , marked with numbers 3 to 6. In this way, droplet ejection is performed repeatedly from the nozzles marked 1 to 6 in sequence from the timings t1 to t6, and one line of a dot row is formed in the main scanning direction on the recording paper 16 by one cycle of droplet ejection in which the droplets are ejected successively from the nozzles marked 1 through to the nozzles marked 6.
Furthermore, in FIG. 24, the symbol P0 indicates the pitch between nozzles that eject droplets to form dots that are mutually adjacent in the main scanning direction. P1 indicates the pitch between nozzles that are mutually adjacent in the main scanning direction (in other words, the pitch between nozzles that eject droplets at the same timing, and the pitch between nozzle 51-16 and nozzle 51-26 in FIG. 24, for example). P2 indicates the pitch between return positions (nozzle row joint sections) (the interval or pitch between return positions that are mutually adjacent when the return positions A are projected to align in the main scanning direction).
The “return position A” is a boundary (joint section) of nozzle rows, each of which is formed by six nozzles aligned in an oblique direction (for example, the nozzles 51-11 to 51-16). In FIG. 24, for instance, the “return position A” corresponds to a region where the pitch in the sub-scanning direction between the nozzles forming dots that are mutually adjacent in the main scanning direction, such as the pitch between nozzle 51-16 and nozzle 51-21 and the pitch between nozzle 51-26 and nozzle 51-31, is greater than the pitch between other nozzles.
FIG. 25A shows a situation where an ink droplet 501 ejected from the nozzle 51-11 at the timing t1 shown in FIG. 24 and an ink droplet 502 ejected from the nozzle 51-12 at the timing t2 shown in FIG. 24 have landed on the recording paper 16. FIG. 25B shows the cross-sectional shape (the cross-sectional shape in the main scanning direction) of the liquid droplets that have landed on the recording paper 16 as shown in FIG. 25A. The reference symbol 511 shown in FIG. 25B indicates an ink droplet (a dot) ejected from the nozzle 51-21 at the timing t1.
As shown in FIG. 25A, the ink droplet 501 ejected from the nozzle 51-11 at the timing t1 lands independently on the recording paper 16, and therefore, no other ink droplets that have already landed on the recording paper 16 affect the ink droplet 501. Consequently, a dot is formed to the original size, at the original landing position d1. In contrast, when the ink droplet 502 ejected from the nozzle 51-12 at the timing t2 is deposited onto the recording paper 16, it is attracted toward the ink droplet 501 (in the direction indicated by the arrow K), due to landing interference (deposition interference) with the ink droplet 501 that has been deposited previously. As a result, a dot is formed at a position d2′ that is shifted toward the ink droplet 501 with respect to the original landing position d2.
In other words, although the pitch (distance) between the dot formed by the ink droplet 501 and the dot formed by the ink droplet 502 is originally intended to be the substantially same as the nozzle pitch P0 between the nozzle 51-11 and the nozzle 51-12, the pitch (distance) between the dot formed by the ink droplet 501 and the dot formed by the ink droplet 502 is in fact a distance of P0'that is smaller than P0 due to the landing interference (aggregation).
Furthermore, the dot formed by the ink droplet 501 absorbs liquid from the ink droplet 502, and therefore, the dot formed by the ink droplet 501 is formed to a greater size than the original size and also has a higher thickness than the original thickness. On the other hand, the dot formed by the ink droplet 502 is formed to a smaller size than the original size, and the thickness of the dot formed by the ink droplet 502 is lower than the original thickness.
Similarly, due to landing interference with an adjacent droplet that has been deposited previously, the ink droplets ejected at the timing t2 to the timing t5 form dots in positions that are near the previously deposited ink droplet compared to their respective original landing positions.
On the other hand, as shown in FIGS. 26A and 26B, the ink droplet 506, which is ejected from the nozzle 51-16 (the nozzle corresponding to the return position) at the timing t6, lands between the ink droplet 505 ejected from the nozzle 51-15 at the timing t5 and the ink droplet 511 ejected from the nozzle 51-21 at the timing t1. Hence, the ink droplet 506 is attracted by both the ink droplet 505 and the ink droplet 511.
More specifically, the ink droplet 506 forms a dot at a position d6′ where the forces of attraction from the ink droplet 505 and the ink droplet 511 are balanced. Provided that the ink droplet 505 and the ink droplet 511 exert substantially the same force of attraction on the ink droplet 506, then the dot is formed in the original landing position by the ink droplet 506.
If the ink droplet 506 forms a dot at an intermediate position between the ink droplet 505 and the ink droplet 511, then the dot pitch P0″ between the dot formed by the ink droplet 506 and the dot formed by the ink droplet 505, and the dot pitch P0″ between the dot formed by the ink droplet 506 and the dot formed by the ink droplet 511 will be greater than the dot pitch P0′ between the respective dots formed by the ink droplets 501 to 505 (in FIGS. 26A and 26B, this corresponds to the dot pitch between the dot formed by the ink droplet 504 ejected at the timing t4, and the dot formed by the ink droplet 505) (i.e., P0′<P0″).
Thus, in the nozzle arrangement shown in FIGS. 22A, 22B, and 23, variation arises in the dot pitches when printing is performed at the regions corresponding to the return position A shown in FIG. 24, and highly conspicuous unevenness in the resulting image may arise in the sub-scanning direction as a result of this variation in the dot pitch. Due to the effect of the peculiarity in landing interference at the regions corresponding to the return position A, the return positions form a peculiar point in the landing interference, and therefore, striped unevenness or non-uniform density is liable to occur in the sub-scanning direction.
FIG. 27 shows a visibility curve 600. The visibility curve 600 shows the boundaries of visibility of uneven density, wherein the horizontal axis denotes the spatial frequency and the vertical axis denotes the density differential (ΔD). In the region above the visibility curve 600, unevenness in density is readily visible, and in the region below the visibility curve 600, unevenness in density is not readily visible. The value lp/mm (line pair/millimeter) indicates the number of dark and light shades per unit length (the number of sets of dark and light shading per unit length).
According to this visibility curve 600, the visibility of uneven density is particularly high at 30 npi to 50 npi (1.2 lp/mm to 2.0 lp/mm), and hence it is especially prominent in the intermediate density region when an image of high density and high quality is formed. According to the general dimensions of the print head 500 described above, the nozzle pitch P1 between nozzles that are adjacent in the main scanning direction is 0.846 mm, which corresponds to around 1.2 lp/mm when converted to the spatial frequency. Hence, the visibility of uneven density is high when the dots are formed by the ink droplets ejected at an interval of P1. In particular, due to the peculiarity at the regions corresponding to the return positions A, it is difficult to increase the spatial frequency of the uneven density to high frequency level of which the visibility is low.
Moreover, although accuracy to approximately 1 μm is required in the diameter and positioning of the nozzles having an aperture diameter of 30 μm, the striped unevenness is liable to become even more conspicuous at the return positions A because lack of continuity in the manufacturing process is liable to arise. This is because the nozzle pitch P02 between the nozzles on both sides of the return position A shown in FIG. 24 is different to the nozzle pitch P0 in other sections, and/or there is disparity in the nozzle diameter. Furthermore, in order to increase yet further the density of the nozzle rows obtained by projecting the nozzles to align in the main scanning direction, it is necessary to increase the number of the nozzles aligned in the main scanning direction. There are possibilities that manufacturing variations will increase in the head overall, while production yield declines.
In the inkjet printer described in Japanese Patent Application Publication No. 2002-166543 and the inkjet head described in Japanese Patent Application Publication No. 2001-334661, the nozzle density is 30 npi to 50 npi, since the actual size of the pressure chamber is approximately 300 μm to 500 μm. If this head is to be used as a line head for high image quality, in particular, then a matrix arrangement of 10 to 20 rows, or greater, (namely, 300 npi to 1000 npi, or greater), is required. Accordingly, a level of difficulty of manufacturing a large head with the nozzles arranged in a high density is raised.
Furthermore, the image forming apparatus described in Japanese Patent Application Publication No. 2002-337320 discloses split adjustment of the head for the purpose of improving the overall production yield and the maintenance performance. However, this splitting of the head is made in the main scanning direction, and the application does not disclose an effective method for achieving high density by means of a matrix arrangement.
Furthermore, the inkjet recording apparatus described in Japanese Patent Application Publication No. 7-17034, and the inkjet printer apparatus and print head unit described in Japanese Patent Application Publication No. 8-25635 have compositions in which joint positions are staggered in relation to respective colors, and beneficial effects will not be expected in monochrome printing.